Projecting Global Mercury Emissions and Deposition Under the Shared Socioeconomic Pathways

Mercury (Hg) is a naturally occurring element that has been greatly enriched in the environment by human activities like mining and fossil fuel combustion. Despite commonalities in some carbon dioxide (CO2) and Hg emission sources, the implications of long-range climate scenarios for anthropogenic Hg emissions have yet to be explored. Here, we present comprehensive projections of anthropogenic Hg emissions extending to the year 2300 and evaluate impacts on global atmospheric Hg deposition. Projections are based on four Shared Socioeconomic Pathways (SSPs) ranging from sustainable reductions in resource and energy intensity to rapid economic growth driven by abundant fossil fuel exploitation. There is a greater than two-fold difference in cumulative anthropogenic Hg emissions between the lower-bound (110 Gg) and upper-bound (235 Gg) scenarios. Hg releases to land and water are approximately six times those of direct emissions to air (600-1,470 Gg). At their peak, anthropogenic Hg emissions reach 2,200-2,600 Mg a-1 sometime between 2010 (baseline) and 2030, depending on the SSP scenario. Coal combustion is the largest determinant of differences in Hg emissions among scenarios. Decoupling of Hg and CO2 emission sources occurs under low-to mid-range scenarios, though contributions from artisanal and small-scale gold mining remain uncertain. Future Hg emissions may have lower gaseous elemental Hg (Hg0) and higher divalent Hg (HgII), resulting in a higher fraction of locally sourced Hg deposition. Projected reemissions of previously deposited anthropogenic Hg follow a similar temporal trajectory to primary emissions, amplifying the benefits of primary Hg emission reductions under the most stringent mitigation scenarios. Mercury (Hg) is a global pollutant that is emitted alongside greenhouse gases like carbon dioxide (CO2) when fossil fuels such as coal are burned. Researchers have projected how emissions of greenhouse gases and climate are likely to change in the future, but relatively little is known about future Hg releases. Here, we project Hg emissions between 2010 and 2300 using growth scenarios developed by the climate change research community. Under low emission scenarios, Hg emissions are projected to peak between 2010 and 2030. Under the high emission scenario, Hg releases continue near present-day levels until after 2060, and decline more slowly than other scenarios thereafter. Large variability in projected releases (cumulatively a two-fold difference) is apparent across the low and high scenarios. Globally, the intensity of coal combustion and how quickly it is phased out is the largest driver of future Hg releases. We used global models to simulate future atmospheric Hg deposition, identifying multiple factors responsible for changing deposition patterns. We find there is a penalty for delaying reductions in Hg emissions because of increased reemissions from the land and ocean in the future. This work emphasizes the benefits of stringent near-term global reductions in anthropogenic Hg releases. Future (2010-2300) anthropogenic Hg releases to air, land, and water (0.7-1.7 Tg) are similar to historical (1510-2010) releases (1.1-2.8 Tg) Cumulative future anthropogenic Hg emissions to air (2010-2300) vary by a factor of two across scenarios (110-230 Gg) Deposition declines from near-term reductions in anthropogenic Hg emissions are amplified by reductions in reemissions from land and the ocean